It is known that a lead sheet, a steel sheet, a laminate consisting of one or two wood sheet and a lead sheet, and a heavy concrete plate are useful for shielding a large dose of radiation. However, the above-mentioned conventional radiation-shielding materials are inconvenient to shield a small dose of radiation because they are too heavy and difficult to be processed. Also, as the above-mentioned conventional radiation-shielding materials are opaque, they cannot be utilized in any operation in which the radiation-shielding material is required to be transparent. Instead, for such operations a lead glass is used. However, the lead glass is disadvantageous in that it exhibits an unsatisfactory mechanical strength and a poor workability, for example, in cutting.
In order to eliminate the above-mentioned disadvantages, several approaches have been proposed to utilize a plastic material containing lead as a radiation-shielding material. That is, the plastic material is utilized as a matrix material having a satisfactory workability. For example, Japanese Patent Application Publication (Kokoku) No. 35-2360(1960) and Japanese Patent Application Laying-open (Kokai) No. 53-9995(1978) disclose a method for producing a radiation-shielding plastic material, in which method lead methacrylate or lead acrylate, alone or together with a vinyl ester monomer, is polymerized. Japanese Patent Application Laying-open (Kokai) No. 53-9994(1978) discloses a radiation-shielding plastic material comprising a blend of a transparent thermoplastic material and a specific organic acid lead salt. Japanese Patent Application Laying-open (Kokai) No. 53-9996(1978)discloses a process in which lead methacrylate or acrylate and a lead salt of a specific organic acid are copolymerized together with a vinyl monomer to provide a radiation-shielding plastic material. Also, Japanese Patent Application Laying-open No. 53-63310(1978) discloses a process in which a vinyl monomer, methacrylic or acrylic acid and a specific organic acid are copolymerized in the presence of lead monoxide, to provide a radiation-shielding plastic material.
However, the above-mentioned prior arts are disadvantageous for the following reasons.
(1) For any particular thickness of the radiation-shielding plastic material, an increase in the radiation-shielding property of the plastic material causes the transparency of the plastic material to undesirably decrease.
(2) The above mentioned increase in the radiation-shielding property of the plastic material also causes the mechanical properties to undesirably decrease, so that the plastic material is easily broken or cracked when it is used. In Japanese Patent Application Laying-open (Kokai) No. 54-1797(1979), an approach is proposed for eliminating the disadvantage in the mechanical properties of the radiation-shielding plastic material. However, this approach is disadvantageous in that the materials to be used are very special and the polymerization process is very complicated.
It is known that methacrylic acid and acrylic acid are effective for holding lead in an effective form for shielding-radiation, in the resultant polymerization product. However, it is also known that when a large amount of lead is held only by the methacrylic acid and/or the acrylic acid, the resultant polymerization product exhibits a decreased degree of transparency and a poor mechanical strength.
Furthermore, it is known that carboxylic acids having a hydrocarbon radical which has 4 or less carbon atoms, exhibit a satisfactory activity for dispersing and dissolving the lead compound in the polymerization mixture. However, since this type of carboxylic acid has a low boiling point, the polymerization mixture containing this type of the carboxylic acid must be polymerized in a restricted temperature range. Also, the resultant polymerization product exhibits a poor thermal property.
Moreover, it is known that other carboxylic acids having a hydrocarbon radical which has 5 or more carbon atoms, exhibit a poor activity for dispersing and dissolving therein the lead compound. Therefore, it is difficult to obtain a polymerization product containing the lead compound in an amount sufficient for obtaining a desired level of radiation-shielding property, and having satisfactory transparency and mechanical properties, from a polymerization mixture containing at least one member of the above-mentioned types of carboxylic acids.
Under the above-mentioned circumstances, it has been strongly desired to provide a process for producing a plastic material which contains therein lead in an amount sufficient for obtaining a desired degree of radiation-shielding activity, and which has excellent transparency and mechanical properties.